Invasive species are those plants and animals that are not native to an area and which have a negative impact on native species, the environment, or human health. Invasive species can also have negative economic impacts due to their interactions with economically important species and local businesses. Since 1999, a non-native marine animal, the Asian green mussel, Perna viridis (Figures 1 and 2), has been found in numerous locations in Florida, Georgia, and South Carolina (Figure 3). Green mussels have the potential to displace local native species such as oysters and other benthic (bottom-dwelling) invertebrates. Furthermore, green mussels are foulers of seagoing ships, stormwater drains, and the intakes of power plants and other industries.
The native range of the green mussel is ocean waters of the Indo-Pacific region from India to Southeast Asia. It has been introduced to coastal waters of Australia, Japan, parts of the Caribbean, South America, and the southeastern United States.
Green mussels were first observed in the Caribbean in 1990. Within 10 years, green mussels had been found along the coasts of Venezuela and Jamaica and in Tampa Bay, Florida. Since their discovery in Tampa Bay in 1999, other populations have been found in coastal regions of southwestern Florida and along the Atlantic Coast of Florida from Palm Beach County northward. A few recruits have been found in Florida in the northern Gulf of Mexico, but no significant populations have yet been reported in that location. Green mussels were first reported in coastal Georgia in 2003; by 2006 they had spread as far north as Charleston, South Carolina. Many of the established populations of P. viridis are in major ports. This suggests that the introductions may have been through ballast water or from mussels attached to the hulls of boats or ships. The introduced population at Tampa Bay has expanded considerably since 1999, most likely through local reproduction and settlement.
The green mussel is a large mussel, 80–100 mm (3–4 inches) in length, occasionally reaching 165 mm (6.5 inches). The shell has a smooth exterior surface which is covered by a thin, organic layer called the periostracum. The periostracum is generally intact in young mussels but may have patches peeled off in older ones. The color of the periostracum is bright green in juveniles, fading to brown with green edges as the mussel matures. The inner shell surface is smooth and iridescent with a bluish-green hue. Green mussels are not to be confused with native mussels. Florida has several species of native mussel, including the scorched mussel, Brachidontes exustus, and ribbed mussel, Geukensia demissa, both of which have ribbed shells, and the horse mussels, Modiolus spp. None of the native mussels has a green color on the lip of the shell (Figure 4).
Green mussels are filter-feeding bivalves. They attach to surfaces using byssal threads. These strong threads allow the mussels to adhere to surfaces, and also allow the mussels to re-attach themselves if they become dislodged. Individual animals are either male or female. Reproduction is by spawning followed by external fertilization. In Hong Kong, these mussels have two peaks in reproduction; one in the spring and one in the fall, although there are some mussels that contain gametes regardless of the time of year. Green mussels in Florida appear to have a similar reproductive pattern. Larvae are planktonic and remain swimming in the water column for 2–3 weeks. At the end of this period, the larvae settle out of the water column, metamorphose into juvenile mussels, and attach themselves to a hard surface. In their native environment of Southeast Asia, green mussels become reproductive at an age of only 2–3 months (20–30 mm shell length, Figure 5). Green mussels may live as long as three years.
Green mussels are generally found growing on hard surfaces such as rocks, pilings, and floating docks; however, they can also be found growing on sandy or muddy bottoms. They seem to prefer to settle in areas where there is high water flow, at a depth of about 30 cm (1 foot) below the low tide mark. They often settle in cryptic areas (e.g., in crevices between boulders or on the lower side of floating objects).
Green mussels can tolerate a fairly wide range of salinities 15–45 ppt and temperatures 50°F–108°F (10°C–42°C).
Green mussels will foul ships, marinas, buoys, and power plant intakes (Figure 6). Young green mussels settled on filters in intake pipes at an electric power plant in Tampa Bay in 1999, clogging the filters and hampering proper operations of the plant. They caused similar problems for a Tampa Bay desalination plant in 2003. Green mussels may clog crab traps and clam culture bags, making the commercial harvest of these native species more difficult. Green mussels compete with other benthic marine species for space. In Tampa Bay, green mussels have displaced some oyster reefs. For recreational boaters who keep their vessels in wet slips, the rapid growth of green mussels in parts of Florida requires an increase in the frequency of hull cleanings. These observations suggest that the biological and economic impact of the green mussel introduction to Florida may be considerable.
Green mussels are eaten by native Florida species including blue crabs and sheepshead; however, the levels of predation are not high enough to substantially reduce or eliminate populations of the mussels.
What can you do?
It is very important that people not facilitate the spread of green mussels to new locations because of the environmental and economic problems they may cause. Recreational and commercial boaters should be aware that green mussels may grow on the hulls of boats that are kept in the water. Before making long-distance trips on your boat, you should have the hull visually inspected, and scraped if necessary, to remove green mussels or other non-native fouling organisms. Ideally, the boat should be hauled and cleaned at a boatyard facility that can retain the scrapings on land. To prevent transporting larvae that may be present in boat bilge water, boaters with trailered boats should drain their bilges and livewells on land when removing their boats from the water.
Green mussels are similar to the New Zealand green-lipped mussel, Perna canaliculus, which is sold in seafood markets in the United States. It is illegal to harvest green mussels from Florida waters that are closed to shellfish harvest (see http://shellfish.floridaaquaculture.com/seas/seas_statusmap.htm for maps). In waters that are open for shellfish harvest, green mussels may only be legally harvested recreationally with a fishing license or by people who have a Commercial Products License from the state. As an invasive species, little is known about product safety of green mussels in Florida however, these mussels are known to accumulate toxins and illness-causing plankton in other parts of the world.
Florida anglers report that they have some success using green mussel meat to fish for saltwater and estuarine sportfish, including sheepshead. Because the mussel tissue is soft, it may be better used as chum rather than as bait on a hook.
For more information: UF/IFAS Green Mussel website http://sfrc.ufl.edu/greenmussel/